11 replies to this topic

[dhns]

Dear Professionals,

Could you please anybody Explain about the issus "When we will go for FC (fail close ) type control valve in Flare line from Seperator (gas high) to Flare header" and When we will go for FO (fail Open ) type control valve in Flare line from Seperator (gas high) to Flare header.

What are the factors to be considered for the same ?

thansk in advance,
REgards,
Dhns.

[djack77494]

Always when considering how an actuated valve should fail, we are attempting to define the position less likely to be damaging, dangerous, or catastrophic to life and property. We are dealing with much uncertainty in that we do not know why the valve failed, what the conditions of the process are at the time of failure, and innumerable other pertinent facts. Despite our ignorance, we must do our best to ascertain what position the valve should move to in the event of the unknown failure(s). So you choose a failure position that you feel is likely to be the lesser of the evils associated with the failure(s), and then get confirmation of your decision during a hazards review of the process.

[Qalander (Chem)]

Always when considering how an actuated valve should fail, we are attempting to define the position less likely to be damaging, dangerous, or catastrophic to life and property. We are dealing with much uncertainty in that we do not know why the valve failed, what the conditions of the process are at the time of failure, and innumerable other pertinent facts. Despite our ignorance, we must do our best to ascertain what position the valve should move to in the event of the unknown failure(s). So you choose a failure position that you feel is likely to be the lesser of the evils associated with the failure(s), and then get confirmation of your decision during a hazards review of the process.

Dear Doug, Well Said/Explained
I feel, there are many possible scenarios
to be envisaged in details during hazops and analytical approach is needed to decide about
which of the many possible consequences are least hazardous/risky to Humans, Property, nearby property and
at times litigation and/or damage(s) to repute, Goodwill are also to be considered in details.
Hope this helps
Regards
Qalander

[fallah]

Normally the control valves connecting the columns,separators,...to flare system are PV and their main functions is controlling the internal pressure of the relevant equipment at normal value.But in hydrocarbon services,those valves should be connected to flare system based on safety requirements.

Therefore they would function in process control system rather than safety system,even though their outlets should be connected to flare.

Those valves are normally closed and their safe position in the case of failure occuring would be usually FC to prevent sudden rising the flare load.Protection of the equipment against overpressure left to alarm and trip in process safety system and finally by PSV,BDV,..

Hope the above can help

[Qalander (Chem)]

Normally the control valves connecting the columns,separators,...to flare system are PV and their main functions is controlling the internal pressure of the relevant equipment at normal value.But in hydrocarbon services,those valves should be connected to flare system based on safety requirements.

Therefore they would function in process control system rather than safety system,even though their outlets should be connected to flare.

Those valves are normally closed and their safe position in the case of failure occuring would be usually FC to prevent sudden rising the flare load.Protection of the equipment against overpressure left to alarm and trip in process safety system and finally by PSV,BDV,..

Hope the above can help
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Even Connecting all such streams outlets to the flare system can heavily increase
Overall Loads and

consequently increasing the size of Flare piping and other systems;rendering these beyond economic viability to erect and operate.

This is a very complicated and thoughtful analytical decision making exercise in itself.

Hope this proves helpful.
Best regards
qalander

[JoeWong]

There are substantial of discussion on this topic within this forum. Please search.

Factors to be considered :

i) Safety
ii) Environment
iv) Health i.e. noise, toxic gas release, etc
iii) Impact to system operation and equipment
iv) Losses related to release of inventory
.
.
.

[ogpprocessing]

Those valves are normally closed and their safe position in the case of failure occuring would be usually FC to prevent sudden rising the flare load.

Can we rely on closed fail safe position of control valves to reduce flare network load?

[fallah]

Can we rely on closed fail safe position of control valves to reduce flare network load?

Why not?

[ogpprocessing]

Can we rely on closed fail safe position of control valves to reduce flare network load?

Why not?

Because fail safe position of control valve is some kind of active protection and all of active type protections may face to malfunction.

[fallah]

Because fail safe position of control valve is some kind of active protection and all of active type protections may face to malfunction.

There could be proper safeguards such as:

-Low pressure alarm,
-Alarm on PV on loss of close condition

[ogpprocessing]

There could be proper safeguards such as:

-Low pressure alarm,
-Alarm on PV on loss of close condition

Flare network is part of pressure relief system as part of last layer of protection. So I think we can not rely on these alarms and safeguards because of probable malfunction. Do you think alarm on PV on loss of closed condition can prevent flare network from overloading?

[fallah]

Flare network is part of pressure relief system as part of last layer of protection. So I think we can not rely on these alarms and safeguards because of probable malfunction.
Internal mechanical stop to be provided at around 60% of design flowrate
Do you think alarm on PV on loss of closed condition can prevent flare network from overloading?
Yes,of course along with the mentioned mechanical stop